Hi Gary,
Your aspen.Tree.FindNode("\Data\Streams\COM1\Output\STR_MAIN\MASSFLMX\MIXED") is a comp object with its attributes, such as Name, Elements, Value... You can check if they exist with the function:
comp = aspen.Tree.FindNode("\Data\Streams\COM1\Output\STR_MAIN\MASSFLMX\MIXED")
hasattr(comp, Elements)
This will return True or False. Also you can loop in this attributes:
for obj in comp.Elements:
if obj.Name == "WATER":
print(obj.Value)
Good luck with your work!